Most of the time when I worked there a BG service is a "poke and go" so they just take a reading from the flue gas and go.
They have missed a number of issues.
1, cant have any sort of isolating valve between the boiler and expansion vessel safety relief valve.
2, no vertical flue support fitted
3, the flue clamp doesn't look like its been drilled and screwed
I'd be asking for a full service, So at the very least, New Burner door seals (ideally would be done every 3 years on these) removing and flushing out the condensate trap, checking and setting the gas/air mix, releasing the system pressure and charging the expansion vessel) That will negate the most likley faults that can be with good servicing, and go some way to ensuring the boilers running safe and well. The full flue in its entirety should be checked and if thats all the flue that you have could be screwed properly with a support fitted in 10 minutes with the right tools and support, and the valve handles removed to prevent accidental shut off at least. Its also a requirement under the british standards now to check/test inhibitor levels in the system and typically renew inhibitor every 5 years.
If they did all the above for £200 then that wouldnt be a terrible deal for london prices.
Go on the vaillant website and find a local vaillant installer, they would likley be best placed to service it, vaillant would aswell, but they will only work on the boiler so may well not check expansion vessel, and could just leave a warning notice regarding the flue rather than correcting it to make it safe.